A two wire sensor is commonly used to sense a condition and to transmit a measure of the sensed condition over two wires to a controller or indicator. The two wire sensor is typically supplied with a voltage V.sub.S over two wires, and the two wire sensor controls the supply current I.sub.S in response to the sensed condition. This supply current I.sub.S is detected by a controller in order to control a load, and/or the supply current I.sub.S is detected by an indicator in order to give an indication of the condition being sensed.
Existing current sources for two wire sensors exhibit several problems. For example, fluctuations in the supply voltage V.sub.S results in corresponding fluctuations in the supply current I.sub.S. Because such fluctuations of the supply current I.sub.S are not related to the condition being sensed, the output of the two wire sensor is not an accurate representation of the sensed condition. Also, existing current sources are sensitive to temperature. Therefore, if temperature is not the condition being sensed, the output of the two wire sensor may fluctuate with temperature changes producing an inaccurate indication of the condition being sensed.
Moreover, variations in the current drawn by the transducers of prior art two wire sensors, as well as by the circuitry associated with the transducers, can also produce inaccurate indications of the condition being sensed. A transducer and its associated circuitry of a two wire sensor are referred to herein as a sensor load.
The present invention is directed to an arrangement which solves one or more of the problems of prior art two wire sensor current sources.